The present invention relates to metallic alloys, and more particularly to alloys including boron and produced using a plasma or flame spray process.
Heretofore, electrical heating elements have had relatively low specific electrical resistances. This has resulted in relatively high cost and lower reliability of heaters.
Using conventional heater materials like NiCr or Kanthal, a high resistance may be achieved in a small volume by using fine wire. A thin wire allows the use of a relatively short wire to achieve the desired electrical resistance. This achieves a small volume heater and also saves material. A problem with this approach, however, is how to transfer the heat developed in the wire to the ambient. Wires have a small surface area, because of their circular cross-section. This results in a relatively high thermal resistance. Thermal resistance may be defined as the ratio of the temperature difference developed across a piece of material, to the thermal power being transferred through that piece. Thus, lower thermal resistance is the capability to transfer more heat energy, while developing a smaller temperature differential.
Systems now in use, including small heaters with fine wires and having a high thermal resistance, are used to deliver relatively high electrical power.
The above structure results in a high temperature differential, that is, the heater wires are very hot, for example about 600-700EC.
A disadvantage of this structure is that expensive wire materials and isolators are used, which are capable of operating at these elevated temperatures.
Another disadvantage is the lower reliability of the heater element, resulting both from high operating temperature (where oxidation is accelerated) and the fine structure of the heater wire (only a small defect in a thin wire can bring immediate local overheating and damage to the heater). Minor imperfections in the heater ribbon may result in local overheating and/or the interruption of the electrical current.
Another approach to heater design is to use amorphous ribbons. It is difficult, with amorphous ribbons, to achieve a heater with small dimensions, since the specific resistivity of the usable metals is relatively low, and the cross section has to be larger than a minimum value. Therefore, the relatively high value of the desired resistance is achieved by using a longer ribbon. Lower operating temperatures may be used, because of the large surface area of these heaters, with corresponding low thermal resistance.
The heater element, however, may not be made in a small size if a relatively high power is required. Moreover, these heaters require means to attach to a substrate, both for mechanical support and to achieve low thermal resistance to that substrate, to deliver the heat developed therein. Amorphous heaters must be operated at a low temperature, less the embrittlement or crystallization temperature is reached, where the amorphous heater material irreversibly changes its structure to become crystalline, the electrical resistance decreases, and the heater may be damaged.
It would be advantageous to make a small heater structure, that is, a heater with small dimensions. It is more convenient to have a small heater, to save space in the house.
Smaller size reduces cost. It also saves raw materials, when one considers the huge market for heating devices.
Smaller size devices, when reduced to waste, have a less detrimental effect on the ecology, since far smaller quantities of waste are produced.
Moreover, existing heaters have a rather complex structure, requiring corresponding complex production methods.
For example, in heating pipes the structure may include an insulator, together with a heater element and with mechanical means to attach the heater to pipe, etc. This is a costly structure and process.
Electrical heaters are used in a wide variety of applications, like domestic heating, industrial/chemical processes, and much more. Thus the importance of low cost, small and reliable electrical heaters is evident.
In accordance with the present invention, these and other objectives are achieved by providing a heater element fabricated by particle deposition using a plasma or flame spray method.
According to one aspect of the present invention, in a preferred embodiment, small particles of a metallic alloy are deposited onto an isolating surface using a plasma or flame spray process. The high temperature of the plasma results in the melting of the particles, so the particles reach the insulating surface in liquid state. The particles splash on the solid surface and solidify, thus forming a metallic layer.
According to a second aspect of the present invention, in a preferred embodiment, the metallic alloy includes boron, so that the liquid particles in the plasma are coated with oxides, with boron oxide being the predominant component. Apparently, during the spray process the structure of the particles is changedxe2x80x94whereas the solid particles have a homogeneous structure, in the liquid globules the boron mostly moves to the outside surface, and the globules become coated with oxides.
According to a third aspect of the present invention, in a preferred embodiment, the liquid particles are deposited on an insulating layer, for example alumina. Good thermal conductivity is achieved, with low temperature differential between heater and the ambient.
According to a fourth aspect of the present invention, in a preferred embodiment, the isolating layer is deposited using a plasma or flame spray process, prior to depositing the metallic alloy particles.
According to a fifth aspect of the present invention, in a preferred embodiment, the isolating layer includes alumina particles. The heater element is impregnated, for example with a silicon organic compound, to close the pores of the isolator.
According to a sixth aspect of the present invention, in a preferred embodiment, the isolating layer includes a protective coating layer using enamel.
According to a seventh aspect of the present invention, in a preferred embodiment, an abrasive with improved mechanical hardness may be obtained by the application of particles of a boron-including alloy, using a plasma or flame spray method.
Further objects, advantages and other features of the present invention will become apparent to those skilled in the art upon reading the disclosures set forth hereinafter.